Airplanes are often towed on the ground using elongated tow bars which engage the plane's nose wheel.
Most tow bars have a forward end adapted to be connected to a towing vehicle such as a truck or the like, usually through a hitch, and the rear end of the bar terminates with a cross bar extending transversely to the length of the tow bar in both directions with a pair of rearward extending arms affixed to each outer end of the cross bar. The opposing sections of the trailing ends of each of these arms carry cylindrical bosses which project in opposition to one another and are adapted to extend into the bores formed in the axle end on each side of the nose wheel.
The trailing arms must be separated sufficiently to clear the nose wheel tire supported on the axle. However, since neither the nose wheel configurations used by different aircraft manufacturers or the tires which may be used on a given nose wheel axle are standard, many of the trailing arms on the tow bar cannot be sufficiently separated to clear a tire supported on the nose wheel axle, to avoid damage to the tire during towing, and still allow the axle-engaging bosses fixed to the rear ends of the arms to project inwardly far enough to engage the bores on opposite ends of the axle.